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For anti-syphon valves to properly operate as backflow protection so that sprinkler water can not mix with your drinking water, the valve have to be installed 6" or more HIGHER than THE HIGHEST SPRINKLER HEAD.

The PROPER thing to do would be to replace ALL of your anti-syphon valves with in-line valves, and install a backflow preventer before the valves. Since your valves are lower than your yard, you really only have two choises.

1. Install an RPZ. This is pretty much legal EVERYWHERE, and when installed properly and checked properly, makes your sprinkler system "bomb proof"... you can even inject fertilizer into the irrigation system and be confident that it will not get back to your drinking water.

2. Install a Dual Check Backflow Preventer. However, you can only do this IF your local building codes allow for it AND you are NOT injecting anything into your irrigation system (even then, there are those that argue that a DC is not adiquate). Keep in mind this is NOT simply a dual check valve. The key difference is that a DC backflow preventer has test cocks so that it can be tested and verify that it is properly preventing backflow.

There are other options, such as rerun the mainline to the top of the hill, install a cheaper PVB backflow preventer, and rerun he main line back to the valves. But the cost the PVB and installing additional mainline would offset the cost difference of just installing an RPZ or DC.

Note that the difference between an RPZ and a DC is that a DC can be installed just about anywhere, including burried inside a meter box, provided the test cocks can be accessed. An RPZ has to be installed above ground and usually costs more than a DC.

As a seperate topic, once you have your irrigation install legally, and you would like to avoid having water from the system draining out the lowest sprinkler head, you might be able to install check valves in the nozzles (sort of). If you have Rainbird 1800 series spray bodies, you can buy 1800-SAM spray bodies and exchange the "guts". The SAM series has check valves built into the bottom of it. If you have something else, there might be a similar option.

elevation change backflow prevention

I'm a neophyte here --

My backyard has a slope in the back, with an elevation of about ten feet above my irrigation valves. When the valve for that circuit closes, the water in the lines flows back and out the anti-syphon valve. This creates a significant wet area near my foundation.

I would like to stop this backflow. What is the best ($ and ease) way to stop this?

I've seen check valve sprinkler heads, which I could try. I thought a check valve near the circuit valve would be better, but most of what I've seen is kind of overkill I think, with costs around $100++.